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492
CHEMICAL ENGINEERING
necessary either to calcine the flint clay before incorporating it in the body or to replace it in part by ground waste bricks (grog).
The burning shrinkage in the case of flint clays cannot be entirely attributed to the contraction accompanying vitrification. Considering the purity of these clays it is evident that part of the shrinkage is independent of this factor and must be due to a molecular change of another kind, that peculiar to many typical amorphous
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BALL CLA
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FIG. 7.—Tennessee ball-clay tests.
substances, like alumina, magnesia, zirconia, etc. We may, therefore, ascribe the high burning-shrinkage of flint clays to colloidal volume changes. Geologists are not agreed as to the origin of flint clays, an obscure subject upon which definite information seems to be lacking.
Besides the flint clays proper there are certain dense fireclays of feeble initial plasticity which lack the other characteristics of this class of materials.    Such a clay
r r 0	_^			A/OfiT	HERN	ILLINOIS	CLAY.			
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1
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FIG. 8.—Illinois ball-clay tests.
occurs in Alabama, which evidently had been formed under conditions radically different from those producing flint clays. Hard nodules of amorphous silica (flint) were found to be associated with this clay which in itself is dense, of white color and by fine grinding can be made to attain a fair degree of plasticity.
Flint clays occur in extensive deposits in the upper Mercer and Middle Kittanning formations of Pennsylvania and Maryland, the Sciotoville in Kentucky and the Lower Kittanning and Upper Freeport in Ohio as well as in the pocket formations of the pre-Pennsylvania limestones, 40 to 140 miles west of St. Louis, Mo., probably of cretaceous origin. The Ohio flint clays as a rule are not equal in refractoriness to the others mentioned for which a softening point of cone 33 to 35 is demanded.